The technique described in the specification relates to incoherent semiconductor light-emitting elements having high luminous efficiency.
Superluminescent diodes (hereinafter referred to as the “SLDs”) have received attention as an incoherent light source that is required in the field of optical measurement such as a fiber gyroscope and medical optical coherent tomography (OCT). The SLDs are semiconductor light-emitting elements using an optical waveguide, as is the case with semiconductor laser diodes (hereinafter referred to as the “LDs”). In the SLDs, spontaneous emission light produced by recombination of injected carriers is amplified with high gain due to stimulated emission while traveling toward a light-emitting end facet, and is emitted from the light-emitting end facet. The SLDs are different from the LDs in that the SLDs suppress formation of an optical cavity due to end facet reflection so that no laser oscillation with the Fabry-Perot (FP) mode occurs. Thus, like normal light-emitting diodes, the SLDs have an incoherent property and a broad spectrum profile, and are capable of providing output of up to about several tens of milliwatts. Accordingly, the SLDs are used in optical fiber gyroscopes etc. In particular, the SLDs using a nitride semiconductor (GaN) have been expected to be applied to video projection systems such as laser displays, as a high output incoherent light source of a visible range from ultraviolet to green (F. Feltin, A. Castiglia, G. Cosendey, L. Sulmoni, J. F. Carlin, N. Grandjean, M. Rossetti, J. Dorsaz, V. Laino, M. Duelk, and C. Velez, Applied Physics Letters, 2009. Vol. 95, No. 8, p. 081107).
FIG. 6A is a schematic diagram showing a conventional GaN-based SLD described in F. Feltin, A. Castiglia, G. Cosendey, L. Sulmoni, J. F. Carlin, N. Grandjean, M. Rossetti, J. Dorsaz, V. Laino, M. Duelk, and C. Velez, Applied Physics Letters, 2009, Vol. 95, No. 8, p. 081107. FIG. 6B is a graph showing a wavelength spectrum of emitted light of the SLD. Mode reflectance is reduced by tilting a ridge optical waveguide at several degrees with respect to a light-emitting end facet. The SLD has a device structure similar to that of a semiconductor laser except that the optical waveguide is tilted with respect to a substrate end facet, namely principal surfaces of an active layer and a cladding layer are tilted with respect to a principal surface of a GaN substrate.